Comparative Study of Pt/Pd and Pt–Rh/Pt Thermocouples

Abstract

The Pt/Pd thermocouple has demonstrated superior thermoelectric drift and homogeneity performance over conventional Pt-Rh/Pt thermocouples. Here, we present a systematic comparison of the drift and homogeneity performance of Pt/Pd and Type R thermocouples by ageing the thermocouples at 1350 • C for a total of 500 h and measuring the performance at regular intervals during this time. The thermocouples studied were one Pt/Pd thermocouple, one Type R thermocouple and one 'special' Type R thermocouple which was given the same preparatory annealing treatment as the Pt/Pd thermocouple prior to use. The thermoelectric stability of each thermocouple was measured at the freezing point of Ag (961.78 • C) and the melting point of Co-C eutectic (1324.29 • C). The thermoelectric homogeneity of the thermocouples was also measured. Two difference methods were used by withdrawing the thermocouple from the Ag cell and by moving a localized heat source along the thermocouple. The long-term drift of the Pt/Pd thermocouple was around 50 mK (Ag) and 65 mK (Co-C) after the first 100 h ageing at 1350 • C, followed by a further 25 mK (Ag) and 35 mK (Co-C) over the subsequent 400 h ageing. This drift performance and inhomogeneity were an order of magnitude lower than for the two Type R thermocou-123 Int J Thermophys (2010) 31:1506-1516 1507 ples. The Type R thermocouple which was given the 'special' preparatory treatment was about 50 % more stable than the conventional Type R thermocouple.

A comprehensive survey of thermoelectric homogeneity of commonly used thermocouple types

“…Type R Data in the literature for the unused inhomogeneity of Type R thermocouples is broad, with a range of measuring temperatures from 185 °C to 1324 °C. The plot in figure 1 was drawn using the results from the literature survey [11,[21][22][23][24][25][26][27][28]. Furthermore the comprehensive survey of Bentley [29] has also determined that the homogeneity of new Type R and S thermocouples exceeds 0.02% of the electromotive force (emf) in only 5% of cases; this result has been widely used in the provision of advice concerning homogeneity.…”

“…The literature on inhomogeneity of Pt/Pd thermocouples is the most extensive, and has a range of measuring temperatures from 140 °C to 1492 °C. The plot in figure 1 was drawn using the literature results [20,21,27,28,30,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. Only the NATA recommendations are shown [1] as the Euramet recommendations [2] are based on tolerance limits for letter-designated thermocouples, and there is not one for the Pt/Pd thermocouple.…”

Investigation of differently designed Pd–C eutectic fixed-point cells for the calibration of thermocouples

“…The melting temperature of the Pd-C eutectic amounts to a value of about 1492 • C [1]. This temperature is near the upper temperature limit of the operation of Pt/Pd thermocouples, which are the most stable thermocouples usable for the measurement of temperatures up to 1500 • C [2]. Pt/Pd thermocouples are less susceptible to thermoelectric inhomogeneities than are for instance Pt-Rh alloyed thermocouples because they are not affected by selective volatilization and oxidation of alloy components [3].…”

Pt-Pd system: Investigation of mechanical properties

“…Zhang et al [19] de-veloped carbon-supported Pd-Pt nanoalloy with low Pt content for formic acid electro-oxidation. Edler and Lehmann [20] studied the mechanical stability of Pt/Pd thermocouples while Ongrai et al [21] conducted comparative study of Pt/Pd and Pt-Rh/Pt thermocouples stability and homogeneity. Ramirez Caballero and Balbuena [22] investigated surface segregation phenomena in Pt-Pd nanoparticles and Hansen et al [23] examined alloy formation and surface segregation in zeolite-supported Pt-Pd bimetallic catalysts.…”